The present invention relates to an exhaust gas recirculation system for an internal combustion engine, and, more particularly, to an exhaust gas reprocessing/recirculation system having variable valve timing.
An exhaust gas recirculation (EGR) system is used for controlling the generation of undesirable pollutant gases and particulate matter in the operation of internal combustion engines. Such systems have proven particularly useful in internal combustion engines used in motor vehicles such as passenger cars, light duty trucks, and other on-road motor equipment.
EGR systems primarily recirculate the exhaust gas by-products into the intake air supply of the internal combustion engine. The exhaust gas which is reintroduced to the engine cylinder reduces the concentration of oxygen therein, which in turn lowers the maximum combustion temperature within the cylinder and slows the chemical reaction of the combustion process, decreasing the formation of nitrous oxides (NOx). Furthermore, the exhaust gases typically contain unburned hydrocarbons which are burned on reintroduction into the engine cylinder, which further reduces the emission of exhaust gas by-products which would be emitted as undesirable pollutants from the internal combustion engine.
Some internal combustion engines include turbochargers to increase engine performance, and are available in a variety of configurations. When utilizing EGR in a turbocharged diesel engine, the exhaust gas to be recirculated is preferably removed upstream of the exhaust gas driven turbine associated with the turbocharger. In many EGR applications, the exhaust gas is diverted by a poppet-type EGR valve directly from the exhaust manifold. The percentage of the total exhaust flow which is diverted for introduction into the intake manifold of an internal combustion engine is known as the EGR rate of the engine.
The reintroduction of exhaust gases will occur naturally when the exhaust manifold pressure is higher than the turbocharger boost pressure. In a low pressure system, the pressure difference simply pushes the exhaust gas into the intake air before the turbocharger compressor. The disadvantage of this approach is the potential fouling of the turbocharger compressor and the air-to-air intercooler of the engine, if so equipt.
High pressure systems typically pump exhaust gas directly into the intake manifold of the engine. However, when such a turbocharged engine operates under lower speed and high torque conditions, the boost pressure is higher than the exhaust manifold pressure and recirculation of exhaust gases is not possible. Earlier approaches to address this problem have included using devices such as back pressure valves, restrictive turbines, throttle valves and venturi inlet systems. Each can be used to improve the back pressure to boost pressure gradient to some degree, but each approach results in increased fuel consumption.
Another approach in reducing exhaust gas emissions, as disclosed in U.S. Pat. No. 5,404,844, is to use a camless engine valve systems and operating method to eliminate the need for external exhaust gas recirculation. In such a system, the engine valves are hydraulically or electrically controlled to vary the valve lift schedule for various engine operating conditions. During part load operation of the engine, the intake valve is opened and the exhaust valve is closed during the exhaust stroke, prior to a piston""s top dead center position, so that the intake port receives exhaust gas, which is then returned to the cylinder during the intake stroke to eliminate the need for an external exhaust gas recirculation system. Such an approach, however, can be limited in its ability to supply a full range of EGR to the engine.
The present invention is directed to overcoming one or more of the problems or limitations set forth above.
In one aspect of the invention, an internal combustion engine has a block defining a plurality of combustion cylinders, the plurality of combustion cylinders having a first cylinder having at least an exhaust stroke followed by an intake stroke. A cylinder head is in fluid communication with the first cylinder, the cylinder head having a first exhaust valve corresponding to the first cylinder and a first intake valve corresponding to the first cylinder. An intake manifold is fluidly connected to the plurality of combustion cylinders to supply combustion air thereto. An exhaust manifold is fluidly connected to the plurality of combustion cylinders to receive exhaust gas therefrom. An apparatus controls a timing of the closing of the first exhaust valve during the exhaust stroke of the first cylinder to trap exhaust gas in the first cylinder for mixing with the combustion air in the first cylinder during the intake stroke.
Thus, the invention provides EGR to an internal combustion engine without the addition of special EGR hardware.